fix(spans): Fix relative paths

[jjbayer]

Turns out the rust schema resolver cannot handle relative paths with a dot, see #360.

Fix relative paths and add a line to rust tests that would have caught this.

ref: INGEST-540

[github-actions]

versions in use:

The following repositories use one of the schemas you are editing. It is recommended to roll out schema changes in small PRs, meaning that if those used versions lag behind the latest, it is probably best to update those services before rolling out your change.

latest version: 2.1.4

There were changes to the JSON schema file, but we couldn't categorize any of
them. Therefore we don't know whether this change is safe to make.

This might be a gap in linting. Want to take a look at
https://github.com/getsentry/json-schema-diff/ and figure it out?

[linear]

INGEST-540 Clean up sentry-kafka-schemas